Process of producing an electrical hesistor

ABSTRACT

AN ELECTRICAL RESISTANCE ELEMENT PARTICULARLY ADAPTED FOR USE IN PRECISION POTENTIOMETERS AND RELATED APPLICATIONS WHEREIN THE HIGH TEMPERATURE RESISTANT AND ESSENTIALLY ELECTRICALLY NON-CONDUCTIVE BASE HAS FUSED THEREON A RESISTANCE MATERIAL COMPRISED OF POWDERED GLASS AND METAL RESINATE SUITABLY TREATED AND ADMIXED WITH PRECIOUS METAL POWDERS WITH WHICH IN THE MIXING PROCESS IS COMBINDED AN ORGANIC VEHICLE PREFERABLY PROVIDED BY ETHYL CELLULOSE DISSOLVED IN AN ALCOHOL EXEMPLIFIED BY 1-UNDECANOL OR 1-DECANOL OR EQUIVALENTS THEREOF. THE INSTANT INVENTION AS APPLIED TO A CERAMIC SUBSTRATE ASSURES UNIFORMLY SUCCESSFUL RESULTS IN THE PRINTING PROCESS, AND AMONG OTHER ADVANTAGES CONTRIBUTES SIGNIFICANTLY TO INCREASED SHELF LIFE FOR THE MATERIAL BEING APPLIED.

March 20, 1973 A. J. STANKAVICH PRO ESS OF PRODUCING AN ELECTRICAL RESISTOR Original Filed Sept. 20, 1968 POWDERED GLASS FIG. 2

ORGANIC MEDIA i MIX I HEAT GRIND suave SINTER SIEVE MIX PRINT ELEMENTS AND FIRE I MEASURE ELECTRICAL CHARACTERISTICS RESINATE METAL POWDERS INVENTOR. ANTHONY J. STANKAVICH ATTORNEY.

United States Patent 27,603 PROCESS OF PRODUCING AN ELECTRICAL RESISTOR Anthony J. Stankavich, Syracuse, N.Y., assignor to Carrier Corp., Syracuse, N.Y. Original No. 3,619,287, dated Nov. 9, 1971, Ser. No. 761,184, Sept. 20, 1968. Application for reissue Dec. 8, 1971, Ser. No. 205,863

Int. CI. 13441! I/18; C23c 17/00 US. Cl. 117227 3 Claims Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE An electrical resistance element particularly adapted for use in precision potentiometers and related applications wherein the high temperature resistant and essentially electrically non-conductive base has fused thereon a resistance material comprised of powdered glass and metal resinate suitably treated and admixed with precious metal powders with which in the mixing process is combined an organic vehicle preferably provided by ethyl cellulose dissolved in an alcohol exemplified by l-undecanol or l-decanol or equivalents thereof. The instant invention as applied to a ceramic substrate assures uniformly successful results in the printing process, and among other advantages contributes significantly to increased shelf life for the material being applied.

BACKGROUND OF THE INVENTION It is known in the art to provide resistor pastes for application by screen printing or like techniques to a base typified by a ceramic substrate. An exemplary procedure is to first prepare a glass-metal resinate by coating glass in particulate form with precious metals such as gold, iridium, ruthenium and rhodium. The glass-metal resinate may then be blended with other precious metals in powdered form, and suitable for this purpose are palladium and silver. The next step in a typical process is to mull the described mixture with an organic vehicle to form a paste having a viscosity suitable for screen printing upon the ceramic substrate or by other modes of application.

However, it has been found after substantial experience that at least two problems are presented, which in production operations have detracted from the commercial success of the process described. First, it has been noted that the prior art organic vehicle when compounded with the other ingredients tend to, in production runs, dry out on the screen or lose other desirable screening characteristics. Second, the volatile liquid carriers previously employed have resulted in a viscosity of the paste lower than that desired, with the consequence that the solid particles in the paste tend to settle out, and the paste accordingly has a relatively short shelf life.

SUMMARY OF THE INVENTION The present invention is directed particularly to a method of preparation of a resistor paste utilizable in the production of electrical resistance elements designed for such exemplary applications as potentiometers. In the process to be disclosed in further detail hereinafter, a glass-metal resinate made from powdered glass and a combination of precious metals is mixed with powders of other precious metals, and is then mulled with a novel vehicle preferably comprised of ethyl cellulose and a straight chain alcohol particularly selected for its proper- Re. 27,603 Reissued Mar. 20, 1973 BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an exploded perspective view illustrating one application for the present invention; and

FIG. 2 is a flow diagram illustrating a preferred process for practicing the novel concepts of this invention.

DESCRIPTION OF THE PREFERRED [EMBODIMENT Thick film resistor pastes find numerous uses in the manufacture of electrical components typified by capacitor electrodes, conductors and resistors. A particularly important application for resistance elements is in precision otentiometers, whether they be of the linear, fixed or rotary type. An illustrative structure embodying the novel concepts of this invention is portrayed in FIG. 1, and a linear potentiometer as shown therein is designated in its entirety by the numeral 10. The potentiometer 10 may comprise a base member 12 constructed to include a support portion 14 mounting in directly afiixed relation thereto a casing 16 which slidably receives for attachment by glue or like means a resistance element 18 to which are directed leads 18a and 18b.

The potentiometer 10 further includes an intermediate portion or collector strip 20 of channel-shape configuration when viewed in cross-section having a longitudinally extending opening 20a formed therein, which when the potentiometer is assembled, provides exposure to coating C on the resistance element 18. As also appears in FIG. 1, the collector strip or buss bar has wired thereto a connection or lead 20b. Constructed for location in surmounting relation to the base member 12, the resistance element 18 received therein, and the collector strip is a wiper housing 22.

The housing 22 embodies a body portion 22a threaded at opposite ends to receive screw means 22b mounting for movement therealong a wiper member 22c equipped with a blade or like device 22d. The body portion 22a is desirably grooved as at 22c to accommodate the leads 18a, 18b and 20b when the parts comprising the assembly 10 are attached one to the other by suitable fastening means. Since the elements of the assembly as thus far described, with the exception of the resistance member 18 and the coating C thereon, are known to the art, further explanation is believed unnecessary.

While the resistance element 18 and the coating C thereon may be produced by various processes, a preferred approach to follow is that outlined in the fiow diagram of FIG. 2. A first step in the method of this invention is to admix a predetermined quantity of a lead borosilicate glass with a precious metal resinate solution made up of a combination of gold, rhodium, ruthenium and iridium. The glass desirably is of the high melting point type, becoming molten at about 800 C., and is ball milled with methanol or other carrier for a sufilcient period of time to pass a 325 mesh sieve. Typically, the glass and metal resinate admixture may comprise ob-out 795.60 grams of glass, 187.20 grams of gold resinate, 93.60 grams each of rhodium and ruthenium resinates, and 30.60 grams of iridium resinate.

The glass-metal resinate mixture after blending is heated to about 400 F. for approximately 45 minutes and after cooling is ground or crushed for about 60 minutes until the majority of the resultant particles are less than of an inch in diameter. The particles obtained are then ieved on a 325 mesh screen, the particles which pass herethrough are heated or sintered for about 40 minutes it approximately 840 F., and then resieved.

At this stage of the process it has been conventional to ombine the glass-metal resinate with a volatile liquid :arrier, and to add one or more precious metals. A com- 1011 organic carrier is butyl carbitol acetate; however, ompounds of this character suffer from several disadantages, the foremost of which is the inability of the lass-resinate mixture when formulated with the organic arrier and additional precious metals to lay down durug the screening process. This prime deficiency of prior art trganic vehicles is believed to be attributable to the relaively low viscosity of the suspension media heretofore mployed, coupled with a rather high vapor pressure and high level of moisture absorption. Further, many of the rganic vehicles heretofore utilized produce during the .ring operation carbonaceous residues which result in 'ariations in the resistivity values of the coated cermet lements within the length or circumference thereof.

To obviate these shortcomings, a novel organic media r volatile liquid carrier is employed in the process porrayed in FIG. 2. The vehicle preferably comprises [poly- 'inyl alcohol or] ethyl cellulose, [the latter being presenty preferred,] dissolved in straight chain alcohol exemlified by l-undecanol or l-decanol or equivalents theref. A typical formulation for the organic media is about grams of ethyl cellulose of the low viscosity type in 80 grams of undecyl alcohol, the former desirably being dded in at least two measured amounts to the solvent with stirring and the dispersion heated at about 150 F. or three to four hours. The organic media, after cooling, then combined with the glass-resinate solution as preiously prepared and these materials are mixed in the eneral proportions of about 180.00 grams glass-resinate material, 14.82 grams silver powder, 5.18 grams pallaiium powder and 70 grams organic vehicle. The mixture s then mulled and subjected to a blending operation for pproximately three minutes.

The product obtained by the procedures thus far decribed is next applied to a siliceous base, designated at ,8 in FIG. 1. The base or substrate is desirably ceramic nd preferred materials are steatite, fosterite, sintered or used aluminas and zircon porcelains. Screen printing or .ke techniques are employed to apply the mixture to the ubstrate, and a typical firing cycle for fusing to the ceamic the product described in the preceding paragraph a from about 15 to 20 minutes at a temperature in the ange of approximately 1400 to 1800 F. Naturally, a urnace may be utilized having stepped temperature zones herein and the speed of travel of the coated element hrough the oven is controlled to assure a firm bond.

Subsequent to the printing and firing step portrayed 1 FIG. 2, measurements are made of the electrical charcteristics to assure effective performance of the coated ermet element when installed in a production environment of the character shown in FIG. 1. Utilizing known nalytical techniques, it has been found that a resistance lement as produced in accordance with this invention ossesses a resistance per square of up to about 200,000 hms, a resistance tolerance of approximately i5%, a :mperature coefiicient of :QOOOOSO ohm per ohm per 4 degree 0, and a power dissipation of about 40 watts per square inch of resistance surface area. Also characterizing the resistance element as herein provided are the properties of fulfilling an operating temperature range within --55 C. to C. and a resistance linearity of 1.0% when divided by element length in inches.

of equal importance, a resistance element 18 having a coating C thereon overcomes the disadvantages of prior art materials by reason of better screening properties and much longer shelf life. The vapor pressure of the alcohol employed is not more than 0.01 mm. Hg at 25 C., and the organic media has a relatively high viscosity, a low afiinity for moisture, and when fired on the ceramic base, there is essentially complete volatilization of the vehicle with the result that carbonaceous residue is substantially absent.

Various modifications may of course be practiced in the formulations and process steps herein disclosed without departing from the spirit of the invention or the scope of the subjoined claims.

I claim:

1. A process of making an electrical resistance element which comprises the steps of forming an intimate, finely divided powdered mixture of glass and metal; forming an organic media consisting essentially of the named components by dissolving [a material selected from the group consisting of] ethyl cellulose [and polyvinyl alcohol] in a straight chain alcohol selected from the group consisting of l-undecanol and l-decanol; blending the intimate mixture of finely divided powdered metal and glass into said organic media to produce a mixture of printable viscosity having a relatively low vapor pressure and a low afiinity for absorption of moisture; printing said mixture on a ceramic base in a pattern of predetermined dimension; heating the mixture on the ceramic base to decompose and drive off the organic media and to simultaneously bond the metal and glass mixture to said ceramic base to form a resistance element.

2. A process of making an electrical resistance element as defined in claim 1, wherein the glass comprises a lead borosilicate glass and wherein said metal essentially consists of a metal selected from the group consisting of gold, iridium, ruthenium and rhodium.

3. A process of making an electrical resistance element as defined in claim 1 including the step of additionally blending a particulate precious metal from the group consisting of silver and palladium into the organic media.

References Cited The following references, cited by the Examiner, are of record in the patented file of this patent or the original patent.

UNITED STATES PATENTS 3,347,799 10/1967 Wagner 252-514 3,385,799 5/1968 Hoffman 252-514 ALFRED L. LEAVITT, Primary Examiner US. Cl. X.R.

117-38 R, 123 B, 124 C, 125, R, 212, 229 

